1. Field of the Invention
The present invention relates to estimation of frequency error caused due to oscillator differences between an IEEE 802.11a based Orthogonal Frequency Division Multiplexing (OFDM) transmitter and an OFDM receiver.
2. Background Art
Local area networks historically have used a network cable or other media to link stations on a network. Newer wireless technologies are being developed to utilize OFDM modulation techniques for wireless local area networking applications, including wireless LANs (i.e., wireless infrastructures having fixed access points), mobile ad hoc networks, etc. In particular, the IEEE Standard 802.11a, entitled “Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High-speed Physical Layer in the 5 GHz Band”, specifies an OFDM PHY for a wireless LAN with data payload communication capabilities of up to 54 Mbps. The IEEE 802.11a Standard specifies a PHY system that uses fifty-two (52) subcarrier frequencies that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Hence, the IEEE Standard 802.11a specifies an OFDM PHY that provides high speed wireless data transmission with multiple techniques for minimizing data errors.
A particular concern in implementing an IEEE 802.11 based OFDM PHY in hardware involves providing a cost-effective, compact device the can be implemented in smaller wireless devices. Hence, implementation concerns typically involve cost, device size, and device complexity.
FIG. 1 is a diagram of a typical direct conversion receiver. The direct conversion receiver 10 includes an antenna 12, a low noise amplifier 14, a local oscillator 16 tuned to a prescribed carrier frequency, mixers 18a and 18b, and lowpass channel filters 20a and 20b. As recognized in the art, I and Q channel signals are generated based on modulating a signal by a first carrier and a second carrier phase-shifted by π/2 (i.e., 90 degrees), respectively. The received signal is supplied to the mixers 18a and 18b. The mixer 18a outputs a first demodulated signal that includes the I component and a first carrier component (e.g., a sine wave); the mixer 18b, having received a phase-shifted carrier signal from the phase shifter 22, outputs a second demodulated signal that includes the Q component and a second carrier component (e.g., a cosine wave). The low pass filters 20a and 20b remove the respective carrier components and output the I and Q components, respectively.
A particular concern involves frequency differences (fE) between the transmit frequency (fT) generated by local crystal oscillator in the OFDM transmitter and the receive frequency (fR) generated by the local crystal oscillator 16 in the OFDM receiver 10. The resulting frequency error (fE=fT−fR) may cause substantial deterioration of the signal to noise ratio of left uncorrected.
The IEEE Standard 802.11 specifies a short preamble and a long preamble that may be used by the OFDM receiver 10 for generating an estimated frequency error (fEST). In actual implementation, however, the estimated frequency error (fEST) does not equal the actual frequency error (fE) because both the short preamble and long preamble contain noise components from transmission between the OFDM transmitter and the OFDM receiver 10. Hence, the short preamble and long preamble received by the OFDM receiver differs from the short preamble and long preamble output by the OFDM transmitter.
Further, the short and long preamble output by the OFDM transmitter do not address frequency errors encountered due to oscillator drift, where the transmit frequency (fT) and the receive frequency (fR) independently vary over time. Hence, the difference between the estimated frequency error (fEST) and the actual frequency error (fE), referred to as residual frequency error (fRES=fEST−fE), still will affect system performance and reduce the signal to noise ratio of the OFDM receiver.